1. Field of the Invention
The present invention relates to a feedback control device and related method for a power converter, and more particularly, to a feedback control device and related method for generating a feedback signal according to a voltage on an auxiliary winding of the primary side of a power converter and generating a reference voltage according to the feedback signal.
2. Description of the Prior Art
A switching power supply (SPS) is used to convert AC power into DC regulated power for use by electronic equipment, and is widely used in a computer, an office automation system, industrial equipment and communications equipment. A power converter in a switching power supply can be of different types, e.g. a flyback converter, a forward converter, and a push-pull converter.
Please refer to FIG. 1, which is a schematic diagram of a power converter 10 according to the prior art. The power converter 10 is a flyback converter and comprises a transformer 100, a switch transistor 102, a pulse width modulation (PWM) control unit 104, an optocoupler 106, and a regulated and error amplifying circuitry 108 in which a shunt regulator diode TL431 is included. The transformer 100 comprises a primary winding Np and an auxiliary winding NA in the primary side and a secondary winding NS in the secondary side for transferring energy and isolating an output terminal and an input terminal of the power converter 10. The PWM control unit 104 generates a pulse signal to control on and off states of the switching transistor 102 so as to control the transformer 100 to transfer a regulated input voltage VIN to an output voltage VOUT. When the switching transistor 102 is turned on, energy is stored in the primary winding Np of the transformer 100; and when the switching transistor 102 is turned off, the energy stored in the primary winding Np is delivered to the secondary winding NS and therefore the output voltage VOUT is generated. Note that when a current flows through the secondary winding NS, variance of the output voltage VOUT is sensed through the auxiliary winding NA.
In order to make the output voltage VOUT stable, a secondary-side feedback control scheme used in the power converter 10 is to amplify error of the output voltage VOUT through the shunt regulator diode TL431 to generate a feedback signal and transfer the feedback signal to the PWM control unit 104 through the optocoupler 106 for performing feedback control. When the output voltage VOUT varies, the PWM control unit 104 adjusts duty cycle of the pulse signal according to the feedback signal to control the switching transistor 102, for regulating the energy delivered to the load of the secondary side of the power converter 10. However, the optocoupler 106 and the shunt regulator diode TL431 are expensive components and occupy a large space in the power converter 10, such that product cost of the power converter 10 cannot be reduced.
Please refer to FIG. 2, which is a schematic diagram of a power converter 20 using primary-side feedback control according to the prior art. The power converter 20 is also a flyback converter and comprises a transformer 200, a switch transistor 202, a PWM control unit 204 and other necessary passive components not described here. Different from the power converter 10, the power converter 20 generates a feedback signal according to a voltage on an auxiliary winding NA instead of using an optocoupler and a shunt regulator diode TL431. When current flows through the secondary winding NS of the power converter 20, variance of the output voltage VOUT is sensed through the auxiliary winding NA. The voltage on the auxiliary winding NA is used as a feedback signal sent to the PWM control unit 204. The PWM control unit 204 adjusts duty cycle of a pulse signal according to the feedback signal to control the switching transistor 202 for regulating energy delivered to a load in the secondary side. Note that the power converter 200 is implemented with many more components than shown in FIG. 2. Even if product cost of the power converter 20 is much less than the power converter 10, it still has a lot of room for improvement.
In order to improve the above drawbacks, Applicant of the present invention has disclosed a power converter with a primary-side feedback control device, as shown in FIG. 3. FIG. 3 is a schematic diagram of a power converter 30 according to the prior art. Feedback control operation of the power converter 30 is similar to that of the power converter 20 except for inclusion of a feedback control device 308. The feedback control device 308 comprises a control unit 310, a comparator 312 and a sample-and-hold unit 314. The comparator 312 is coupled to an auxiliary winding NA of the power converter 30 and is utilized for comparing the voltage on the auxiliary winding NA with a predefined reference voltage VREF for generating control signals for controlling the sample-and-hold unit 314. The sample-and-hold unit 314 is controlled by the control signals to output a feedback signal to the control unit 310. The control unit 310 generates a pulse signal VPWM according to the feedback signal to control on and off states of a switching transistor 306, so the energy is transferred from the primary side to the secondary side.
Note that, when the power converter 30 is turned on at the beginning, LC (inductance-capacitance) ringing occurs on the voltage on the auxiliary winding NA. In this situation, if the predefined reference voltage VREF is non-adjustable and lower than a voltage of the feedback signal, the voltage of the feedback signal may be decreased continuously such that a voltage of the pulse signal VPWM is not large enough to drive the switching transistor 306.
From the above, using the optocoupler and the shunt regulator diode TL431 is an expensive solution for feedback control and the production cost is not reduced. On the other hand, although the primary-feedback control device the Applicant has disclosed has a reduced cost, it cannot prevent influence of LC ringing when the power converter is just turned on, which ultimately results in failure to control the switching transistor.